The invention disclosed and claimed herein deals primarily with the use of magnetic separators for treating silicon-containing materials from a fluid bed reactor to remove magnetically influenced components in the silicon-containing materials. The removal of such components allows for enhanced reactivity of the silicon-containing materials in processes wherein the silicon-containing materials are raw materials for the production of silicon based compounds, such as, for example, basic alkylhalosilanes such as dimethyldichlorosilane, methylhydrogendichlorosilane, and other chlorosilanes such as trichlorosilane, which chlorosilanes are useful in the preparation of valuable silicon-containing products.
As indicated Supra, certain valuable halosilanes, that is, the halosilanes that form essentially the basis for the entire silicone products industry, are produced from the reaction between elemental silicon and an alkylhalide at elevated temperatures in the presence of a copper-based catalyst and various promoters. Other similar reactions are carried out to produce other silanes, for example, the preparation of trichlorosilane, which is a basic building block for the production of silicon metal.
There are literally hundreds of patents and publications directed to the basic reaction to produce the alkylhalosilanes, known in the industry as the Direct Process, the most fundamental and earliest being U.S. Pat. No. 2,380,995, that issued August 1945 to Rochow, directed to the chemical process and U.S. Pat. No. 2,389,931, that issued in November, 1945 to Reed, et al., directed to the fluidized bed reactors in the Direct Process.
The main purpose of the Direct Process is to make dimethyldichlorosilane, however, other silanes are produced such as methyltrichlorosilane, trimethylchlorosilane, tetramethylsilane and methyldichlorosilane, and other chlorosilanes and various methylchlorodisilanes, which find limited commercial use, along with direct process residue which is a combination of numerous compounds which are present in minor amounts and are not essentially commercially useful wherein the residues are high boiling having normal boiling points greater than about 71xc2x0 C. These residual materials are well described in the literature.
There is a constant effort in the industry to enhance the Direct Process so that it is more selective in terms of producing the main component, dimethyldichlorosilane, and is more efficient to provide higher yields at a faster rate. In addition, intimate control of the process is desired such that when compounds other than dimethyldichlorosilane are desired, such as methyldichlorosilane, the process can be controlled to generate these compounds in higher yields.
Unfortunately, the commercial process as currently operated results in less control of the reaction as it proceeds, and this is thought to be due to the accumulation of impurities in the fluid bed reactors as the reaction within the fluid bed reactors progresses. In fact, the process is initially very active and highly selective to products of interest. Over time, performance degrades, allegedly due to the impurity buildup, and thus, the process has to be shut down periodically and the fluid bed contents purged, regenerated or refurbished in order to return the process to an acceptable yield level and rate of reaction, and more importantly, the selective formation of dimethyldichlorosilane. Metallurgical grade silicon typically contains 0.4% weight Fe, 0.15% weight Al, 0.08% weight Ca and 0.03% weight Ti (see U.S. Pat. No. 5,334,738 to Pachaly). The non-silicon metals form a range of intermetallic species such as FeSi2, CaSi2, FeSi2Ti, Al2CaSi2, Al8Fe5Si7, Al3FeSi2, Al6CaFe4Si8, FeSi2.4Al, and the like, which are also described in the open literature.
The selectivity of the formation of the chlorosilanes has been defined by Dotson, in U.S. Pat. No. 3,133,109, that issued May 12, 1964, as the ratio of organotrichlorosilane (T) to diorganodichlorosilane (D) (the T/D ratio), and it is generally desired to have this ratio below about 0.35 The modern objective is to minimize this ratio. Whenever used herein, the term xe2x80x9cdesired ratioxe2x80x9d means the desired T/D ratio.
A further publication regarding the various factors affecting the degree of usage of the silicon in the Direct Process can be found in M. G. R. T. de Cooker, et. al., xe2x80x9cThe Influence of Oxygen on the Direct Synthesis of Methylchlorosilanesxe2x80x9d, Journal of Organometallic Chemistry, 84, (1975), pp. 305 to 316, in which de Cooker discloses that during the Direct Process synthesis, a gradual deactivation of the contact mixture surface occurs. He speculates that this deactivation may be caused by a number of factors. For example, the deposition of carbon and carbonaceous products may block part of the surface. Furthermore, the activity can be decreased by decreasing the content of the promoters on the contact mixture surface per se, for example, as caused by the evaporation of ZnCl2, by the accumulation in the reactor of elements present as contaminants in the silicon, for example, iron, by the increase of free copper on the surface causing enhanced cracking, or by the blocking of the reactive sites by reaction of the contact mixture with traces of oxygen, yielding silicon and copper oxides. Silicon used in the experiments as disclosed in that article was technical silicon, as opposed to metallurgical silicon, wherein the main impurities of the technical silicon were described as being 0.4% weight Fe, 0.1% weight Al and 0.3% weight of each of Ca and Mg, and before use, the silicon was washed with water, dried, and treated with a magnet to remove part of the iron present in the silicon.
Thus, there is a need to overcome the impurity buildup and allow the reaction to run longer, with greater efficiency and increased yields, with better control over the products that are produced. Several references discuss impurities and their removal by withdrawing a stream from the reactor, separating an impurities-lean portion and returning it to the reactor. The term xe2x80x9ccontent ratioxe2x80x9d as used herein is calculated as the ratio of the weight percent of a given element in an impurities-rich fraction divided by the weight percent in an impurities-lean fraction. A content ratio of 1.0 indicates that there are equal concentrations of the given element in rich and lean fractions and thus no separation occurred for that element.
One solution for the removal of the impurities from the fluid bed reactants during the course of the reaction and thus decrease the impurity buildup in the reactors is disclosed in U.S. Pat. No. 4,307,242 that issued to Shah et al. on Dec. 22, 1981 in which a size classification method, e.g., aerodynamic centrifugal classifier, is used.
U.S. Pat. No. 4,281,149, that issued Jul. 28, 1981 to Shade describes a means of abrading a portion of the silicon particles in the reactor so surface poisoning is overcome and fresh reaction surfaces are exposed. Whenever used herein, the term xe2x80x9cabradedxe2x80x9d or xe2x80x9cabrasionxe2x80x9d means the processes set forth in Shade, which disclosure is incorporated herein by reference for what it teaches about the abrasion of solid particles from reactors.
The inventors herein are aware of other disclosures in the prior art that deal with the separation of metals from divided solid materials using magnetic separation technologies. There are two such disclosures from the refinery industry, neither of which deal with the magnetic separation of components from silicon-containing materials. One such piece of prior art is U.S. Pat. No. 5,147,527, that issued Sep. 15, 1992 to Hettinger, which discloses the magnetic separation of high metals-containing catalysts into low, intermediate and high metals, and active catalyst. Thus, the patent describes an improved process for converting carbo-metallic oils into lighter products using catalysts, the enhancement being a process of passing a portion of the catalyst particulates through a high strength magnetic field of at least one kilogauss and field gradients of at least 10 kilogauss/inch while conveying them on an electrostatic conducting belt and recycling the more active catalyst back to the process in which it was initially used.
A second disclosure can be found in U.S. Pat. No. 6,194,337, that issued on Feb. 27, 2001 to Goolsby, et al., in which the magnetic susceptibility of impure particles is enhanced to improve magnetic separation of undesirable contaminants in a catalyst.
Various references describe the application of magnetic forces to remove ferromagnetic and paramagnetic particulate impurities from mine ores and slurries. Svoboda, Jan., xe2x80x9cMagnetic Methods for the Treatment of Mineralsxe2x80x9d, Developments in Mineral Processing-8xe2x80x9d, ISBNO-44-42811-9, Elsevier, New York, 1987, reviews the state of magnetic separation technology. Other general references include xe2x80x9cMagnetic Separationxe2x80x9d, Perry""s Chemical Engineers"" Handbook, McGraw-Hill, New York, 7th Edition, 1998, pp. 19-49 and Oberteuffer, John, Wechsler, lonel, xe2x80x9cMagnetic Separationxe2x80x9d, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, 1978, John Wiley and Sons, New York, Volume 15, pp.708-732. These references describe the technology of the induced magnetic roll separator, the permanent magnetic roll separator, the high gradient magnetic separator (HGMS), and open gradient magnetic separator, all of which are useful in the instant invention.
Some applications of magnetic separation have been demonstrated in silicon related chemistry. Wang, et al., in Magnetic and Electrical Separation, xe2x80x9cPurification of Fine Powders by a Superconducting HGMS with Vibration Assistance, Vol. 10 (2000), pp. 161-178 demonstrate the ability of an HGMS to remove Fe2O3 from quartz. Seider, et al., in U.S. Pat. No. 4,810,368, that issued on Mar. 7, 1989 shows the beneficial separation of magnetic impurities from silicon carbide. Barraclough, et al., in U.S. Pat. No. 5,349,921, that issued Sep. 27, 1994, improved the impurity distribution in semiconductor grade silicon with the application of a 500 gauss magnetic field during crystal growth. Wiesner, in U.S. Pat. No. 6,264,843, that issued on Jul. 24, 2001, teaches how to remove impurities from the machining of semiconductor material wherein particles from saw blades or lapping plates can be magnetically separated from the cutting fluid used during the machining process for silicon.
Various authors have reported on magnetic susceptibility of silicon-containing materials. U. Birkholz, et al., report in Physica Status Solidi, 1969, No.34, pp. K181-K184 the magnetic susceptibility of xcex1-FeSi2 in the temperature range of 0xc2x0 C. to 1000xc2x0 C. The xcex1-FeSi2 has low magnetic susceptibility with a flat response in the temperature range of 0xc2x0 C. to 400xc2x0 C. Small amounts of excess silicon added make the magnetic susceptibility negative, that is, diamagnetic. D. Mandrus, et al., in Physical Review B, Vol. 51. No. 8, February 1995, pp. 4763-4767, report the magnetic susceptibility of FeSi in the temperature range of 50 K to 700 K. The FeSi shows a peak magnetic susceptibility at approximately 225xc2x0 C. However, from the FeSi phase diagram from O. Kubaschewski, Iron- Binary Phase Diagrams, Springer-Verlag, 1982, pp. 136 to 139, and from reported intermetallic phases of commercial grade silicon, FeSi is not expected to be present in the feed silicon for the Direct Process.
None of the above-described references teach, show, or describe the magnetic separation of magnetic influenced species from silicon materials from fluid bed reactors to benefit the production of silanes. Also, none of the above-described references teach, show or describe an optimum temperature for magnetically separating the impurities expected to be present in the silicon.
The processes disclosed and claimed herein control impurity buildup in the fluid bed of the reactor and enhance the reaction therein to provide a more efficient process, better selectivity, better process control and longer run times for the reaction.